1. Field of the Invention
This invention relates generally to the field of geophysical prospecting and more particularly to a method and apparatus for imparting a seismic impulse to the earth's surface utilizing a high speed catching system.
2. Description of the Prior Art
The invention to be described is of primary interest in connection with the control and operation of so-called seismic gas exploders. Typically, these are devices resting on the ground wherein a gas explosion takes place within an expansible chamber formed between a bottom impact mass and a movable piston assembly, causing the impact mass to move downwardly to generate a seismic wave within the earth. The piston assembly moves in an opposite direction, followed by the upward rebound of the entire exploder housing. If the exploder is allowed to fall back to earth under the force of gravity, a secondary signal will be generated upon the subsequent impact which will interfere with the reflection of the original impulse. Thus, it is desirable either to eliminate this secondary signal or to at least minimize its amplitude.
Prior art catcher systems are adapted to allow the gas exploder to rise freely to the top of its rebound and then restrain its return to the earth. This type of catcher sequence may be performed with the aid of at least one upstanding fluid filled hydraulic cylinder provided with a piston to form a dashpot which is suitably coupled to the gas exploder. See, for example, U.S. Pat. No. 3,810,524 to Clifford D. Dransfield. In this type of device, upward movement of the exploder is adapted to move the piston freely until the exploder reaches its maximum rebound height, while reversal of direction of the exploder closes a unidirectional check valve in the piston so that thereafter hydraulic fluid must bypass the piston through one or more metering holes or orifices. The result is that the entire descent of the exploder is slowed, and no significant secondary signal is produced.
Prior art catcher systems as described have been designed which enable an exploder to rise to the top of its travel in about 0.3 seconds while the return to earth requires about 3.0 seconds. It is desirable, however, particularly in repetitive operation of the exploder, such as in the generation of a rapid coded sequence of seismic pulses, to reduce the replacement time of a gas exploder to one second or less provided this can be accomplished without generating false signals.
A further problem in the operation of prior art seismic exploders lies in the fact that an exploder may be fired when it is not in contact with the earth. Many exploders cannot withstand this stress safely. Furthermore, the resultant seismic pulse may differ substantially from one generated when the exploder interfaces the earth. For example, in repetitive operation of an exploder, the earth may be compacted and consequently lowered in level to some extent by the impact of a shot. An automatic timing sequence for firing such a gas exploder at intervals may not take account of such variations. Thus, there is a need to insure that the exploder has returned to earth so that such firing does not occur prematurely. Applicant's copending application Ser. No. 526,841 entitled METHOD AND APPARATUS FOR SENSING WHEN A SEISMIC GAS EXPLODER IS IN CONTACT WITH THE EARTH, and incorporated by reference herewith, is directed to one solution of this problem.